Implants are used both for humans and animals in order to support functions of the bone skeleton or even to replace parts of the skeleton. The implants, which support functions of the skeleton and which can therefore also be described as supporting implants, include plates, splints or angular-stable systems made from a solid material such as, for example, high-quality steel or titanium which are secured to skeleton bones in order to take over their supporting function. Such supporting implants are used, for example, in conjunction with bone fractures in order to fixate bone parts relative to one another during the healing process. In this case, implants are secured to the bones with the help of screws, for example. For the purpose of joining bone surfaces together involving bone fractures or in plastic surgery, special implants are used such as, for example, plates or angular-stable systems. For the fixation of the bones, the bone fragments are joined to the plates by means of screws or bolts in particular. In order to allow an adaptation of the osteosynthesis to the individual patient situation, the plates are provided with several openings for accommodating the screws or bolts.
The healing, for example the healing of a bone fracture, is a biological process whose process depends on numerous biological but also bio-mechanical ambient factors. There are proposals for supporting the healing by means of systemic or local application of stimulating substances. The release of corresponding substances in the timing sequence and the effects on relevant bio-physical and/or bio-chemical parameters at the location of interest can be only insufficiently assessed at the present time. Independent of effected/non-effected support, the progress of the healing process is presently assessed on the basis of methods which have serious disadvantages.
The evaluation of X-ray photos is the standard method for the documentation of the healing progress where bone fracture healing is concerned. However, this method involves a radiation load for the patient and is therefore an invasive method where, in addition, an objectified documentation of the healing process is only restrictively possible. The decision as to when the healing process is completed, to the extent that a temporarily applied implant can be removed again for example, is therefore essentially determined by the experience of the surgeon. Methods for the assessment of the healing progress on the basis of computed tomography are also invasive methods because of the radiation load. For a lying implant the evaluation is normally difficult, but is also intensive with regard to time and resources and involves very high costs, and therefore a routine application is not possible. The same applies for MR-based methods that are time-intensive, not always applicable for a lying implant or have a limited validity of statement, are cost-intensive and cannot be applied in large numbers. For special methods for the stabilisation of fractures, for example with so-called “fixateur exteme”, non-invasive methods are applicable which document in a non-contact manner the change in the relative movement of the bone fragments with the help of optical measurements (G. N. Duda, B. Bartmeyer, S. Sporrer, W. R. Taylor, M. Raschke, N. D. Haas; “Does partial weight bearing unload a healing bone in external ring fixation?”; Langbecks Arch Surg., October 2003, 388(5), 298-304.) and can thus illustrate the course of the healing. For stabilising methods where implants, for example, plates, angular-stable “fixateur interne” lie underneath the skin and the securing elements are not accessible for an optical measurement, the non-contact method as described beforehand is not applicable.
U.S. Pat. No. 6,034,296 describes a sensor system for fixation to an implant. The sensor system is equipped with a pressure-measuring system in order to measure the mechanical stresses and strains occurring in the implant during everyday usage. For this purpose, the sensor system is secured to the implant with the help of screws or by means of welded joints. In addition, the sensor system comprises a telemetry unit for the purpose of transmitting the measured values to an external receiver.
DE 198 58 889 A1 discloses an implant for the fixation of bones. As an embodiment form, it is proposed to arrange on the implant a sensor for the detection of forces acting on the implant as well as a telemetry unit for the transmission of the measuring results to an external unit.